Rotating font discs are widely used for photocomposition machines. The font is a transparency in an opaque background, and the entire series of characters of a single font are in a single circular path of the rotating disc. There are usually several separate font faces on each disc. The disc is caused to rotate with one of the circular paths of font characters located at a position to permit light to project through the transparency characters into an optical system of the machine.
Fonts have been historically produced on rigid glass supports in order to prevent wavering which would throw the character transparency beyond the optical depth of focus of the optical system. A common plane must be established and maintained within the optical perimeters of any optical system in order to provide proper focus and projection of quality images. The problem of maintaining such optical perimeters is often compounded because of the use of highly flexible materials such as thin photographic film for economic as well as quality reproduction reasons.
There are many factors such as handling, material grain, temperature and humidity which affect the flatness of such photographic materials. Also, varying rotational speeds of such materials will result in the formation of excessive dynamic distortion if not properly restricted. Therefore, a guidance system is required. Within such a guidance system, minimum wear of materials and non-clogging characteristics are of utmost importance.